1. Field of the Invention
The present invention relates to a display panel that is a surface device having a plurality of light emitting portions. More specifically, the invention relates to a surface discharge type plasma display panel (PDP) having partitions for defining discharge gas spaces.
2. Background of the Related Art
As a television display device having a wide screen, a 42-inch surface discharging format AC type PDP is commercialized. In this surface discharging format, first and second main electrodes are arranged in parallel on the front or the back substrate (usually, a glass plate). The first and the second main electrodes become anodes and cathodes alternately in the AC drive that sustains the lighted state by utilizing wall charge (charge of the dielectric). In the surface discharging format, compared with another format in which the first and the second main electrodes are arranged to cross each other, a fluorescent layer for color display can be arranged on the other substrate that faces the substrate on which the main electrode pairs are arranged. Thus, deterioration of the fluorescent layer due to ion impact upon discharge can be reduced, and a long life can be achieved. A xe2x80x9creflection typexe2x80x9d in which the fluorescent layer is formed on the back substrate is superior to a xe2x80x9ctransparent typexe2x80x9d in which the fluorescent layer is formed on the front side substrate, concerning a light emission efficiency.
The surface discharging format PDP has partitions each of which is arranged for every column of a matrix display for defining the inner discharge space. The partition separates the discharge connection between neighboring columns, and the size (thickness) of the discharge space is defined. Normally, the partitions are formed on the substrate on which the fluorescent layer is formed. By providing partitions having height corresponding to the thickness of the discharge space on one of the substrates, the registration of a pair of substrates in an assembling process becomes easy compared with the case where partitions having height that is a half of the thickness of the discharge space on both the substrates, for example. In addition, fluorescent layer can be provided so as to cover not only the upper surface of the substrate but also the side face of the partitions, so that the light emitting area can be increased and the viewing angle can be widened.
In the production of PDPs, the material of each member is selected considering affinity with substrate. The above-mentioned partition is formed by burning glass paste that has a low melting point and is applied in a predetermined pattern. The paste layer is formed by using a screen printing method or by cutting out unnecessary portions of a uniform layer.
Conventionally, the partition is colored by mixing a black or a white organic pigment in glass frit that is a main material of the partition, so the partition is substantially opaque. When the partition is colored black, it can absorb visible light more so as to reflect external light less, resulting in good contrast of display. In contrast, if the partition is colored white, it can reflect visible light rays more, so that the light rays that are emitted by the fluorescent layer and are directed to the partition can be recycled to the surface of the fluorescent layer for display use.
However, if the partition is colored black, the light that is emitted by the fluorescent layer and enters the partition can be absorbed by the partition, so a loss of light is generated. If the partition is colored white, external light rays may be reflected by the partition so that the contrast may be reduced. Namely, it is difficult to improve both the contrast and the intensity in the conventional structure.
The object of the present invention is to provide a display panel in which both the intensity and the contrast are improved.
A first display panel according to the present invention is characterized in that partitions are translucent that are elements neighboring a light emitting portion. In addition, a second display panel is characterized in that the partition includes a first layer that is transparent or translucent and a second layer having a large absorption ratio for visible light, and the second layer is disposed at the back side of the first layer.
In the translucent structure, useful light rays that enter the partition (light rays emitted inside the panel) pass the partition and are directed to the front though they are attenuated at a predetermined ratio. In contrast, external light rays that enter the partition propagated inside the partition and are reflected by the bottom surface of the partition to be directed to the front after passing through the partition again.
Compared with the useful light rays emitted inside the panel, the external light rays pass through the partition twice before returning to the front side, so that the ratio of the attenuation of the external light rays due to the pass through the partition is larger than that of the useful light rays. If the reflectivity of the external light (ratio of intensities of the emitting external light and the entering external light) is desired to be 0.1, the transparency xcex1 in the optical path corresponding to the height of the partition should satisfy the following inequality (1).
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Here, xcex2 is the reflectivity of the under layer of the partition. In order to enhance the intensity of the display, transparency xcex1 is desired to be larger. If the reflectivity xcex2 is decreased by coloring the under layer of the partition dark, the inequality (1) will be satisfied and the transparency xcex1 can be increased.
In the double layer structure, the useful light rays pass through the first layer and are directed to the front side. In contrast, the external light that enter the partition pass the first layer and enter the second layer to be absorbed by the second layer. If the refractive index of the first layer is selected to the value substantially the same as that of the second layer, undesired reflection at the interface between the first and the second layers so that the external light rays that return to the front can be eliminated substantially.
According to the present invention, since the absorption of the useful light rays in the partition and the reflection of the external light can be reduced, the intensity can be improved without reducing the contrast, or the contrast can be improved without reducing the intensity. The present invention can be applied preferably to a display panel of a matrix display format in which the partitions occupy a relatively large area in the front surface. However, the present invention can be also applied to a display panel of a segment display format in which specific characters and signs can be displayed. In addition, the format of the light emission is not limited. It can be a self emission format such as electric discharge in a gas and electroluminescence or back light format for LCD.